Warpage Control in Package-on-Package Structures

ABSTRACT

A package includes a bottom substrate and a bottom die over and bonded to the bottom substrate. A metal-particle-containing compound material is overlying a top surface of the bottom die, wherein the metal-particle-containing compound material comprises metal particles. A molding material molds at least a lower part of the bottom die therein, wherein the molding material is overlying the bottom substrate.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/096,456, entitled “Warpage Control in Package-On-Package Structures,”filed on Dec. 4, 2013, which application is incorporated herein byreference.

BACKGROUND

In a conventional Package-on-Package (PoP) process, a top package, inwhich a first device die is bonded, is further bonded to a bottompackage through solder balls. The bottom package may also include asecond device die bonded therein. The second device die may be on thesame side of the bottom package as the solder balls.

Before the bonding of the top package to the bottom package, a moldingcompound is applied on the bottom package, with the molding compoundcovering the second device die and the solder balls. Since the solderballs are buried in the molding compound, a laser ablation or drillingis performed to form holes in the molding compound, so that the solderballs are exposed. The top package and the bottom package may then bebonded through the solder balls in the bottom package.

There is significant mismatch between the Coefficients of ThermalExpansion (CTEs) of the materials in the PoP packages. For example, thepackage substrate and the molding compound have CTEs that are muchhigher than that of the device dies. Accordingly, in the resultingpackage, there is a significant warpage.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 9 are cross-sectional views of intermediate stages inthe manufacturing of a Package-on-Package (PoP) structure in accordancewith some exemplary embodiments.

DETAILED DESCRIPTION

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A package and the method of forming the same are provided in accordancewith various embodiments. The intermediate stages of forming thepackages are illustrated in accordance with some embodiments. Thevariations of the embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

FIGS. 1 through 9 are cross-sectional views of intermediate stages inthe manufacturing of a Package-on-Package (PoP) structure in accordancewith some exemplary embodiment. Referring to FIG. 1, package component10 is provided. In some embodiments, package component 10 is a packagesubstrate, and hence in referred to as package substrate 10 hereinafter,although it may be of another type of package component such as aninterposer or a package. Package substrate 10 may be a part of a packagesubstrate strip (not shown). For example, a package substrate strip mayinclude a plurality of package substrates having identical structures aspackage substrate 10. The package substrates in the package substratestrip may be arranged as an array.

Package substrate 10 may be a build-up substrate or a laminatesubstrate. For example, as a build-up substrate, package substrate 10includes substrate 11 that is formed of a dielectric material, whereinconductive vias 15 penetrate through substrate 11. Package substrate 10also includes metal lines/vias 14 on the opposite sides of substrate 11.Package substrate 10 may also be a laminate substrate, which includeslaminated dielectric films, and conductive traces built in thedielectric films. Package substrate 10 is configured to electricallycouple connectors 12 on the top side of package substrate 10 toconductive features 16 on the bottom side of package substrate 10.Conductive features 12 and 16 may be metal pads, for example.

Electrical connectors 24 are formed on top surfaces 10A of packagesubstrate 10. Electrical connectors 24 are electrically coupled to, andmay be in physical contact with, electrical connectors 12. In someembodiments, electrical connectors 24 are solder balls. In alternativeembodiments, electrical connectors 24 comprise metal pads, metalpillars, solder caps formed on metal pillars, and/or the like. Thesolder regions (such as solder balls or the reflowed solder caps) ofelectrical connectors 24 may have round top surfaces, although the topsurfaces of the solder regions may also be planar.

Referring to FIG. 2, device die 20 is bonded to package substrate 10through electrical connectors 12. Throughout the description, device die20 is also referred to as a bottom device die since it is located in abottom package. Device die 20 may be circuit dies comprising integratedcircuit devices (not shown) such as transistors, capacitors, inductors,resistors, and the like. Device die 20 may be a logic die such as aCentral Computing Unit (CPU) die. Device die 20 may also represent a diestack include a plurality of dies stacked together. The bonding ofdevice die 20 to electrical connectors 12 may be through solder bondingor direct metal-to-metal bonding (such as copper-to-copper bonding). Insome embodiments, top ends 24A of electrical connectors 24 aresubstantially level with, or lower than, top surfaces 20A of device die20. In alternative embodiments, top ends 24A of electrical connectors 24are higher than top surface 20A of die 20.

Referring to FIG. 3, metal-containing paste 26 is applied on top surface20A of device die 20. In some embodiments, the application ofmetal-containing paste 26 is performed through stencil printing, whichincludes placing a stencil 28 over device die 20, with a through-hole 30of stencil 28 being aligned to a center region of top surface 20A.Metal-containing paste 26 is then applied on stencil 28. Excessmetal-containing paste 26 is then wiped out using squeegee 31. Squeegee31 has a planar bottom surface, and hence the portion ofmetal-containing paste 26 left in through-hole 30 has a planar topsurface. After stencil 28 is lifted, a portion of metal-containing paste26 is left in hole 30.

Metal-containing paste 26 may include metal particles mixed with anadhesive. The metal particles may include the particles of a pure metal,a metal alloy, or the like. In some embodiments, metal-containing paste26 is a copper paste including copper particles. The metal particles inmetal-containing paste 26 may have a weight percentage greater thanabout 20 percent, wherein the weight percentage is measured eitherbefore or after the subsequent curing of metal-containing paste 26. Theadhesive in metal-containing paste 26 may include Phenolic resin, epoxyresin, or the like. Metal-containing paste 26 may be electricallyconductive, and hence may adopt the same paste that is used onelectrical connector. In these embodiments, metal-containing paste 26may be a solder paste. Metal-containing paste 26 is applied as asemi-fluid, so that it can be stencil-printed, yet can maintain itsshape before cured. For example, metal-containing paste 26 has aviscosity in the range between about 800 Pa-S and about 1,300 Pa-S. Insome embodiments, the diameters of the metal particles (the filler) inmetal-containing paste 26 is between about 3 μm and about 10 μm, orbetween about 5 μm and about 6 μm. The glass transition temperature Tgof metal-containing paste 26 may be higher than about 100° C. In someexemplary embodiments, the glass transition temperature ofmetal-containing paste 26 is about 110° C.

FIG. 4A illustrates the curing of metal-containing paste 26. Throughoutthe description, metal-containing paste 26 is alternatively referred toas a metal-particle-containing compound material. Stencil 28 as show inFIG. 3 is also removed. In some embodiments, metal-containing paste 26includes thermal curing, for example, by heating metal-containing paste26 to a temperature higher than the glass transition temperature ofmetal-containing paste 26. For example, the curing temperature may bebetween about 150° C. and about 200° C. The curing may be performed fora period of time between about 60 minutes and about 120 minutes,depending on the type of metal-containing paste 26. After the curing ofmetal-containing paste 26, the cured metal-containing paste 26 has aCoefficient of Thermal Expansion (CTE) greater than the CTE of devicedie 20. For example, device die 20 may include a silicon substrate,which has a CTE of about 3.2 ppm/° C. Hence, (the cured)metal-containing paste 26 has a CTE greater than 3.2 ppm/° C.Furthermore, the difference between the CTE of metal-containing paste 26and the CTE of device die 20 may be greater than about 10 ppm/° C.,greater than about 20 ppm/° C., or higher. The Young's modulus ofmetal-containing paste 26 is preferably high, for example, higher thanabout 5×10⁹ Pa. Thickness T1 of metal-containing paste 26 may be greaterthan about 50 μm, so that it can provide enough support to balance thestresses in the resulting package.

The higher CTE (than device die 20) and the high Young's modulus ofmetal-containing paste 26 help balance the stresses in the resultingpackage. Package substrate 10 may have a high CTE, for example, higherthan about 15 ppm/° C. Since device die 20 has a low CTE, the resultingpackage may have a warpage, wherein the center portion of the resultingpackage may by higher than the corner portions and edge portions. Withmetal-containing paste 26 and package substrate 10 (both having CTEshigher than that of device die 20) being disposed on the opposite sidesof device die 20, the stresses in the resulting package (such as thestresses on the opposite sides of device die 20) are balanced, and thewarpage in the resulting package is reduced.

As shown in FIG. 4A, due to the weight of metal-containing paste 26 andits viscosity, the shape of metal-containing paste 26 may changeslightly before and during the curing process. The resulting curedmetal-containing paste 26 may have a profile with the bottom portionslarger than the upper portions. The edges of metal-containing paste 26are also tilted. Hence, metal-containing paste 26 may have a trapezoidshape in the cross-sectional view. Furthermore, the top corners ofmetal-containing paste 26 may be rounded.

FIG. 4B illustrates the package in accordance with alternativeembodiments. In these embodiments, rigid plate 27 is placed overmetal-containing paste 26. Rigid plate 27 may be pre-formed before theplacement, and may have a CTE greater than the CTE of device die 20. TheYoung's modulus of rigid plate 27 is preferably high, for example,higher than about 5×10⁹ Pa. The Young's modulus of right plate 27 mayalso be higher than the Young's moduli of metal-containing paste 26,silicon, and the subsequently applied molding compound 32 (FIG. 5A).Rigid plate 27 may have thickness T2 greater than about 30 μm in someembodiments. In some embodiments, rigid plate 27 includes a copperplate. In alternative embodiments, rigid plate 27 may be another type ofmetal plate including, and not limited to, a stainless steel plate, a Alplate, a Cu plate, or the like. In yet alternative embodiments, rigidplate 27 comprises a non-metal plate, which may include ceramic, forexample. In the embodiments that rigid plate 27 is included,metal-containing paste 26 may act as the adhesive for adhering rigidplate 27 onto device die 20.

The formation of the package in FIG. 4B may include applyingmetal-containing paste 26 on the top surface of device die 20, andplacing rigid plate 27 over metal-containing paste 26. After rigid plate27 is placed over metal-containing paste 26, a curing process isperformed to cure metal-containing paste 26. Metal-containing paste 26thus acts as the adhesive for adhering rigid plate 27 to the underlyingdevice die 20.

FIG. 4C illustrates a top view of package substrate 10, device die 20,and solder balls 24, wherein the top view may be the top view of FIG. 4Aor FIG. 4B. In some embodiments, solder balls 24 are disposedsurrounding device die 20. Metal-containing paste 26 covers a centerportion of device die 20. The edges 26A of metal-containing paste 26 arespaced apart from edges 20B of device die 20 by margin D1, so that theapplied metal-containing paste 26 does not flow over the edges of devicedie 20. In some embodiment, margin D1 is between about 300 μm and about1,000 μm. When the width of device die 20 is denoted as W1, ratio D1/W1may also be between about 0.02 and about 0.08 in some embodiments,although different ratios may be used.

Referring to FIG. 5A, molding material 32 is molded on dies 20 andpackage substrates 10, and is then cured. Molding material 32 may bedifferent from the adhesive in metal-containing paste. In some exemplaryembodiments, molding material 32 comprises a polymer, which may be amolding compound, an underfill, a Molding Underfill (MUF), or the like.Solder balls 24 are buried in molding material 32 in some embodiments.In alternative embodiments, the lower portions of solder balls 24 are inmolding material 32, and the upper portions of molding material 32protrude above molding material 32.

In some embodiments, molding material 32 molds metal-containing paste 26and rigid plate 27 (if any, FIG. 4B) therein. In alternativeembodiments, molding material 32 molds a lower part of device die 20,and the top surface of molding material 32 may be at any level that isunderlying the top surface of metal-containing paste 26 (or rigid plate27 when it exists). For example, the top surface of molding material 32may be at the levels shown as 33, which are marked by dashed lines. Insome embodiments, the top surface of molding material 32 is level withthe top surface of rigid plate 27. In alternative embodiments, the topsurface of molding material 32 is between the top surface and the bottomsurface of rigid plate 27. In yet alternative embodiments, the topsurface of molding material 32 is level with the interface of rigidplate 27 and metal-containing paste 26. In yet alternative embodiments,the top surface of molding material 32 is between the top surface andthe bottom surface of metal-containing paste 26. The top surface ofmolding material 32 may also be level with or lower than the bottomsurface of metal-containing paste 26.

FIGS. 5B and 5C illustrate the magnified views of portion 29 of thepackage in FIG. 5A. As shown in FIGS. 5B and 5C, metal-containing paste26 includes top surface 26B, edge 26A, and bottom surface 26C that is incontact with the top surface of device die 20. Top surface 26B includesa portion that is substantially planar. Edge 26A may also include asubstantially straight portion forming a rounded corner 26D with theplanar portion of top surface 26B. The radius r1 of the rounded corner26D may be in the range between about 20 μm and about 40 μm in someexemplary embodiments. The substantially straight edge 26A ofmetal-containing paste 26 and bottom surface 26C form angle α, which maybe between about 45 degrees and about 75.

In some embodiments, as shown in FIG. 5B, rigid plate 27 is used, andhence the planar top surface 26B is in contact with the bottom surfaceof rigid plate 27. In alternative embodiments, as shown in FIG. 5C,rigid plate 27 is not used, and hence edge 26A is in contact withmolding material 32. Depending on where the top surface of moldingmaterial 32 is located, edge 26A and the rounded corner 26D may, or maynot, in contact with molding material 32B in various embodiments.

FIG. 6 illustrates the exposure of solder balls 24. In some embodiments,the exposure includes a laser trimming step to remove the portions ofmolding material 32 that covers solder balls 24. The energy of the laserused in the laser trimming process is adjusted, so that molding material32 is trimmed when exposed to the laser, while solder balls 24 are notremoved even exposed to the laser. As shown in FIG. 6, as a result ofthe laser trimming, openings 34 are formed, with solder balls 24 exposedto openings 34.

FIG. 7 illustrates the formation of solder balls 36 on metal pads 16.Throughout the description, the structure shown in FIG. 7, whichincludes package substrate 10, device die 20, molding material 32, andmetal-containing paste 26 etc. are in combination referred to as bottompackage 100.

Referring to FIG. 8, top package 200 is placed onto bottom packages 100.Solder balls 210 of top package 200 are aligned to, and are placed on,solder balls 24. In some embodiments, top package 200 includes devicedie 204, and package substrate 206, on which device die 204 is bonded.Furthermore, molding material 208 may be molded on device die 204 toform top package 200.

Next, as shown in FIG. 9, a reflow is performed, and hence top package200 is bonded to bottom package 100. After the reflow, solder balls 24and 210 (FIG. 8) are molten and joined, and the resulting solder regionsare referred to as solder regions 50 in FIG. 8. After the reflow, aunderfill (not shown) may be disposed between top packages 200 andbottom package 100. A sawing step may be performed to saw top package200 and bottom package 100 apart from other parts (for example, therespective package substrate strip). The resulting package is referredto as a PoP package 300.

In the embodiments of the present disclosure, by applying ametal-containing paste that has a CTE greater than the CTE of therespective underlying device die, the difference in the CTEs of thematerials on the opposite sides of the device die reduced, and stressesin the resulting PoP package is reduced. The warpage is the resultingPoP package is also reduced.

In accordance with some embodiments, a package includes a bottomsubstrate and a bottom die over and bonded to the bottom substrate. Ametal-particle-containing compound material is overlying a top surfaceof the bottom die, wherein the metal-particle-containing compoundmaterial comprises metal particles. A molding material molds at least alower part of the bottom die therein, wherein the molding material isoverlying the bottom substrate.

In accordance with other embodiments, a package includes a top packagebonded to a bottom package. The bottom package includes a packagesubstrate, and a device die over and bonded to the package substrate. Ametal-particle-containing compound material is overlying a top surfaceof the device die, wherein the metal-particle-containing compoundmaterial comprises metal particles. A molding compound is overlying thepackage substrate, wherein the molding compound molds the device dietherein. The top package is bonded to the bottom package through solderregions penetrating through the molding compound.

In accordance with yet other embodiments, a method includes bonding adevice die over a package substrate, applying a metal-containing pasteover a top surface of the device die, and curing the metal-containingpaste. After curing the metal-containing paste, the device die is moldedin a molding compound, wherein the molding compound includes a portionoverlying the package substrate.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: bonding a device die over apackage substrate; applying a metal-containing paste onto a top surfaceof the device die; curing the metal-containing paste; and after curingthe metal-containing paste, encapsulating the device die in anencapsulating material, wherein the encapsulating material includes aportion overlying the package substrate, and the encapsulating materialis in contact with at least a lower portion of a sidewall of the devicedie.
 2. The method of claim 1, wherein after the metal-containing pasteis cured, a top surface of the device die is exposed, and a topmostsurface of the encapsulating material is lower than the top surface ofthe device die.
 3. The method of claim 1 further comprising placing arigid non-metal plate over and contacting the metal-containing paste,wherein the cured metal-containing paste joins the rigid null-metalplate to the device die.
 4. The method of claim 3, wherein the placingthe rigid non-metal plate comprises placing a ceramic plate.
 5. Themethod of claim 1, wherein the encapsulating material has a portionhaving a bottom surface in contact with a top surface of themetal-containing paste.
 6. The method of claim 1, wherein the packagesubstrate, the device die, the metal-containing paste, and theencapsulating material in combination form a bottom package, and themethod further comprises bonding a top package to a bottom package. 7.The method of claim 1, wherein the applying the metal-containing pastecomprises applying a copper paste, and the copper paste comprises copperparticles therein.
 8. The method of claim 1, wherein when themetal-containing paste is cured, no rigid plate is located over themetal-containing paste, and a top surface of the metal-containing pasteis exposed.
 9. A method comprising: bonding a device die over a packagesubstrate; applying a metal-containing paste onto a top surface of thedevice die; curing the metal-containing paste; and after curing themetal-containing paste, encapsulating the device die in an encapsulatingmaterial, wherein the encapsulating material includes a portionoverlying the package substrate, with a top surface of the device dieand an entirety of the metal-containing paste being exposed through theencapsulating material, and a top surface of the encapsulating materialis lower than the top surface of the device die.
 10. The method of claim9 further comprising, before the metal-containing paste is cured,placing a rigid non-metal plate over and contacting the metal-containingpaste.
 11. The method of claim 10, wherein the placing the rigidnon-metal plate comprises placing a pre-formed ceramic plate.
 12. Themethod of claim 10, wherein a Young's modulus of the rigid non-metalplate is greater than a Young's modulus of the cured metal-containingpaste.
 13. The method of claim 10, wherein the rigid non-metal plateextends laterally beyond a point where a respective edge of themetal-containing paste joins the rigid non-metal plate.
 14. The methodof claim 9, wherein the applying the metal-containing paste comprises:placing a stencil over the device die, with an opening of the stencilaligned to a center region of the device die; dispensing themetal-containing paste into the opening; and using a squeegee to removeexcess portions of the metal-containing paste.
 15. The method of claim9, wherein after the device die is encapsulated, both a sidewall of themetal-containing paste and an upper portion of a sidewall of the devicedie have portions exposed.
 16. A method comprising: bonding a device dieonto a package substrate; applying a metal-containing paste onto a topsurface of the device die; placing a rigid plate over and contacting themetal-containing paste; with the rigid plate being over themetal-containing paste, curing the metal-containing paste; andencapsulating the device die in an encapsulating material, wherein theencapsulating material includes a portion overlying the packagesubstrate, with a top surface of the device die exposed through theencapsulating material, wherein after the encapsulating, a portion of asidewall of the device die is exposed.
 17. The method of claim 16,wherein after the curing, the metal-containing paste has a rounded topcorner.
 18. The method of claim 17, wherein the rigid plate laterallyextends beyond the rounded top corner, with a portion of a bottomsurface of the rigid plate untouched by the metal-containing paste. 19.The method of claim 16, wherein the metal-containing paste and theencapsulating material are formed of different materials.
 20. The methodof claim 16, wherein a Young's modulus of the rigid plate is greaterthan a Young's modulus of the cured metal-containing paste.